1. Field of the Invention
This invention relates generally to oscillator circuits, and more particularly, to a biasing scheme for reducing the phase noise of an oscillator circuit employing an LC tank.
2. Description of the Related Art
High-frequency oscillators are of fundamental importance in all communications systems. Their most important attributes are: spectral purity; ease of frequency control (including, in some instances, tuning range); power consumption; and ease of integration in monolithic form, since most modern communications systems must be manufacturable at low cost. The present invention addresses all of these imperatives with special emphasis on spectral purity.
The design of high-frequency oscillators based on the use of LC resonant tanks is fairly easy for most routine applications. However, in many modern communication systems such as GSM, the performance requirements are very stringent, and the design is consequently more demanding. In particular, it is essential to produce an output spectrum which is very pure. The idea of spectral purity involves two different and, in principle, unconnected qualities. First, it may refer to the harmonic distortion of the oscillator output. In some applications, such as an oscillator intended for use in driving a mixer in a receiver, harmonic distortion is not of direct importance (though even-order distortion may be troublesome). In other applications, the lack of spectral purity due to the presence of harmonics can be addressed by the addition of filters.
However, filters are ineffective in dealing with the second kind of spectral impurity, generally described as "phase noise". This can be thought of as a smearing of the power spectrum across the frequency domain due to rapid, non-coherent frequency modulation by noise energy associated with the complete oscillator, comprising, in the present context, a passive LC tank circuit coupled to an active driving circuit. The noise in the tank itself is generated by any lossy elements such as the series resistance of the coil (L), dielectric losses in the capacitor (C) and shunt losses in any dissipative load placed across the tank. Often, a varactor diode is used to provide voltage-control of the oscillator, and its series resistance is also a source of noise. However, these sources in themselves are generally not dominant. Rather, they lead to a lowering of the selectivity of the tank, often expressed in terms of the quality factor "Q", and it is this reduced ability of the tank to reject wideband noise from the active elements which is troublesome. The noise power associated with the transistors in the active drive circuit, notably the base resistance of a bipolar transistor, or the channel resistance of an FET, as well as shot noise components, deliver noise to the tank in the process of supporting the oscillation amplitude. Further excess noise occurs whenever these active elements enter a region of overdrive of a particular kind.
FIG. 1 is a schematic diagram of a prior art oscillator circuit having an LC tank and accompanying drive circuit. Although the circuits described herein will be explained with a view to implementation in monolithic integrated circuits for use in radio frequency (RF) oscillators, the principles generally apply in other contexts as well. The circuit of FIG. 1 includes an LC tank which is made up of a center-tapped inductor L, and a capacitor C. A voltage source V.sub.S provides DC power to the circuit through the center tap of the inductor L. The tank is driven by a drive circuit 10, also known as a negative impedance converter ("NIC"), formed by transistors Q1 and Q2, and bias current source CS1 which generates a tail current I.sub.T.
The inductor L and capacitor C usually have parasitic (intrinsic) resistances which can be modeled as resistors connected in series with the inductor and capacitor. However, to simplify the analysis, the parasitics, as well as the load resistance, can be mathematically transformed and combined for modeling as the single resistor R in FIG. 1. The load resistance includes the input impedance seen looking into the input of any circuit the tank is used to drive, for example, a differential pair of transistors. The load resistance also accounts for other loading effects such as losses from dielectric effects, eddie currents in the substrate material, and radiation. For convenience, the effective resistance from all the above-mentioned sources will be referred to simply as the load resistance.
When the circuit of FIG. 1 begins oscillating, the tank voltage causes the tail current I.sub.T to commutate back and forth between nodes N1 and N2 in response to the voltages in the tank, thereby sustaining the oscillation.
A problem with the circuit of FIG. 1, however, is that it is too noisy for use in high-frequency applications where high spectral purity is required.
Another problem with the circuit of FIG. 1 is that its operation is susceptible to variations in the load resistance and component tolerances, as well as variations in the values of the inductor, the capacitor, and their parasitic resistances, caused by semiconductor process variations.
Accordingly, a need remains for a scheme for improving the performance of an oscillator circuit.